Recovery of natural gas from deep brines

ABSTRACT

A method is described for circulating hydrostatically pressured or geopressured brines so that dissolved methane in the brines can be recovered within the wellpipe. All processes take place downhole or in the surrounding briny formations, and the circulation is powered wholly or in large part by the pressure on the brine and natural compressive forces in the formation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The methane-containing, sedimentary formations are drilled to greatdepths, into or in search of gas caps which will deliver natural gas(primarily methane) to the surface. These wells are capped off andabandoned when the gas cap is exhausted or if no gas cap is located.With deep wells, e.g., at depths greater than 10,000 feet, this practiceleaves unrecovered what is usually the major fraction of the natural gaspenetrated. Specifically, it leaves behind (a) natural gas dissolved inthe hot, high-pressured brine and (b) gas trapped as small bubbles inthe pores of the host formation. If the dissolved gas is extracted insome way from its solution in the brine, then the entrapped bubbles willresupply the depleted gas dissolved in the brine, again creating asolution of natural gas in brine. In fact, sampling of the gas solutionsin brine from actual wells confirms that the brine is indeed saturatedwith natural gas, consistent with the presence of both dissolved gas andbubbles in two thermodynamically different phases. Gas which isdissolved or entrapped in small pores is not considered recoverable on acommercial basis.

In explaining this invention it is useful to consider two representativesets of conditions downhole, one involving brine at normal hydrostaticpressures and the other dealing with brine at abnormal pressures becauseof some additional lithostatic pressure, i.e., geopressured. Under eachset of conditions the natural gas is recoverable, but the environmentalproblems are greater for the geopressured case.

First, for the hydrostatically pressured case, assume the followingconditions: the formation containing the brine has a porosity of 25% anda permeability of one darcy, the hydrostatic pressure is about 0.465psia per foot of depth, so a well drilled to 15,200 feet has abottom-hole pressure (BHP) of 7,065 psia. The temperature is 300° F.,and at this temperature the pressure of saturated steam over the brineis about 65 psia. In addition, the brine is saturated with natural gasso that its partial pressure of 7,000 psia plus that of the steam justequals the 7,065 psia BHP. The methane concentration to achievesaturation is 38 standard cubic feet (SCF) per bbl of brine atbottom-hole conditions. The brine is saturated with the solids making upthe host formation, and additional solubility of CaCO₃ and othercarbonates results from the presence of dissolved CO₂ which addsCa(HCO₃)₂ to the solution. Near-saturation quantities of CaSO₄ and othersulfates may be present even though the solids do not exist in theformation. NaCl and other dissolved chlorides are not of greatimportance for the present analysis. Temperatures and pressures decreaseat depths less than those at the bottom of the wellpipe, and thepressures of steam and methane fall to near zero at ground level or theocean surface. In a theoretical sense these conditions are unstablebecause the hot brine could in principle move to a lower pressure regionand discharge methane and steam. Here the hot brine and gases whichcould no longer be contained by the hydrostatic pressure head would rushup the wellpipe in a continuing action much like the action of a coffeepercolator or geyser. In practice there is a vanishingly smalllikelihood that the upward flow of brine would initiate itself; if,however, the system is designed properly, and if the circulation isinitiated, then this tendency to release methane can be used tocirculate brines so that their dissolved methane can be removed in aspecial type of stripper. The potential to release steam must besuppressed because steam vaporization may lead to solid precipitationand plugging of the wellpipe. This invention describes a method toinitiate and continue the circulation while suppressing the steamvaporization and wellpipe plugging.

The work used to circulate the brine is derived from two in situsources, and these in situ sources of work can be supplemented fromexternal sources such as engines at the earth or water surface. First,in situ, simultaneous and coupled release of virgin brine andreinjection of spent brine back into the formation are used to balancethe release and injection forces, and, second, in situ, additionalenergy to overcome frictional forces is available from the release ofmethane and limited amounts of steam as pressure is reduced, and theexpansion of these gases provides a fluid which can do useful workdownhole. If the methane pressure is dropped from 7,000 psia to 14.7psia (atmospheric pressure), then 38 SCF of natural gas per bbl of brinewill be released. About 7.8% of the brine also will boil off before thetemperature drops from 300° F. to 212° F. and atmospheric pressure isreached. This alteration of the brine will result in precipitation ofsolids both because the amount of water is decreased and becausedissolved CO₂ is removed by gas sweeping as steam escapes. If, however,the methane pressure is maintained high enough so that the totalpressure is over 65 psia, then the 300-degree brine cannot boil andsteam removal is greatly reduced. As a corollary little dissolved CO₂escapes and the brine concentrations are not altered so the solutionsremain stable and solids do not precipitate. Furthermore, violentejections of brine will be largely controlled if boiling is prevented.If, for example, the gas pressure is maintained at 100 psia, then over99% of the dissolved methane can be released, and the gas released at300° F. from one bbl of brine will consist of 65 psia of steam plus 35psia of methane jointly making up 100 psia of gas pressure in a totalvolume of 16 cu ft. In this case less than 0.01% of the brine boils awayand no important precipitation of solids occurs. However, the volume ofthe fluid is essentially quadrupled (5.6 cu ft per bbl of brine to 21.6cu ft for brine plus gas), and the gas volume at 100 psia is availableto pump a third as much brine volume at 300 psia for circulation andinjection of spent brine. In this case the brine can be withdrawn from ahot region, circulated through a methane stripper, and reinjected into aslightly different region. Because the formation is porous and thepressure is hydrostatic, brine will flow to equalize pressures rapidly,and there will be no subsidence.

Now consider a geopressured formation in which the pressure is partlyhydrostatic at about 0.465 psia per ft of depth and partly lithostaticat about 1 psia per ft of depth. The well is 15,200 ft deep, thetemperature is 300° F. at the bottom of the hole, the BHP is 12,000psia, and there is a methane solubility of 60 SCF per bbl. Release ofthis methane can produce 25 cu ft of methane plus steam at 100 psia.This gas volume at 100 psia can move the brine volume at about 450 psia.If the brines are stripped of their methane and then reinjected in tothe same geopressured formation but at a remote region, so that removaland reinjection are hydraulically linked, then large regions of theformation can be made available for methane recovery while the chance ofserious subsidence is much reduced. This opportunity for limitingsubsidence in geopressured regions by reinjection of the spent brineback into its original formation is solved by this invention.

To prevent collapse of the wellpipes, it is necessary to design thesystem so that the high pressure differences between the formationpressure and the pressure of the product methane do not act against thewellpipe.

Because cooling the saturated brines can lead to precipitation ofsolids, the cooling should be minimized, thus the methane is extractedin the hot regions of the wellpipe rather than where ocean or groundwater has cooled it.

2. Prior Art

A. "Method and Devices for In Situ Recovery of Gaseous Hydrocarbons andSteam," by G. R. B. Elliott, N. E. Vanderborough, and M. W. McDaniel,Patent application Ser. No. 15,360, filed Feb. 26, 1979. This patentapplication recognizes the value of recovering in situ energy to assistin the reinjection of spent brine after methane removal, and it pointsout the value of recovering the methane without ever bringing brine tothe ocean or earth surface. It does not disclose that the release ofdissolved methane can suppress solids precipitation from steamvaporization while supplying nearly all of the energy needed tocirculate the brine.

B. "The Status of Dissolved Gas in Japan," by S. S. Marsden, inPetroleum Engineer, June 1980, pp. 23-34. This paper describes theJapanese production of methane from methane-containing brines whichinvolves pumping brine to surface facilities where methane is strippedout. The pumps are not self-powered, and no deep, hot wells areinvolved; rather, the wells are typically at 1,500-3,000 feet depthswith 6,000 feet being maximum depth.

C. U.S. Pat. No. 4,149,598 (4/79) and 4,149,596 by Christian et al.These patents show that methane can be recovered from aquifiers whichare hydrostatically pressured and in this recovery, large quantities ofbrine are lifted to the surface by pumping from the surface. The pumpingcauses a pressure drop in the brine near the wellpipe and, so long aspumping is continued, dissolved methane can be released to a gas phase.If pumping is stopped, the pressure of the brine will again rise to thehydrostatic pressure head, and gas evolution will cease. The Christianpatents do not recognize the value of using the expansion of the methaneupon pressure release to drive the pumping necessary to circulate virginbrine into position for methane release, and the serious problem ofdisposing of the spent brine which is brought to the surface.

D. U.S. Pat. No. 3,782,468 by Kuwada. This work identifies that theevaporation of brine to steam can cause the circulation of hot brine tothe surface for processing, and injection of CO₂ is described tosuppress the precipitation of carbonates and hydroxides which could formas steam release swept CO₂ out of solution.

E. U.S. Pat. No. 4,131,161 by Lacquement. This patent describes the useof a standpipe inside a wellpipe to circulate brine and recover drysteam from deep, hot wells. This patent uses in situ forces to achievethe pumping to circulate brine, and all the steam recovery facilitiesare below ground. The work does not address serious limitations imposedon the invention if steam is released from hot, saturated brine, i.e.,geyserlike ejection of brine up the wellpipe and precipitation of solidswith wellpipe plugging.

F. "Petroleum Production Handbook," Editors T. C. Frick and R. W.Taylor, Chapter 6, "Hydraulic Pumps," by C. J. Coberly and F. B. Brown,and Chapter 31, "Wellbore Hydraulics," by J. K. Welchon, A. F. Bertuzzi,and F. H. Poettman. These chapters indicate the pumping and gasliftconcepts which are used in oil and gas production.

OBJECTS OF THE INVENTION

It is an object of this invention to strip methane economically andenvironmentally safely from hot brine at downhole depth and inject thedepleted brine immediately into a formation penetrated by the wellpipe.

It is another object of this invention to use in situ forces tocirculate the brine.

It is a further object of this invention to use downhole pumps orturbines powered by in situ forces to circulate the brine and suppresssteam vaporization and precipitation of solids.

Other objects, advantages and novel features of this invention will beapparent to those of ordinary skill in the art upon examination of thefollowing detailed description of a preferred embodiment of theinvention and the accompanying drawings.

SUMMARY OF THE INVENTION

A method and device for circulating hot, natural brine and recoveringmethane is described. This circulation is driven partially or completelyby in situ forces, once initial pumping has started the device. Thedevice can operate without moving parts, but more efficient operationcan be accomplished if the system uses pumps powered by in situ forces.The principle driving force is the expansion of methane after thehydrostatic pressure on a deep brine has been released. The gas which isreleased suppresses harmful steam vaporization, it lifts brine to astandpipe, it forces spent brine from standpipes and into disposalformations, and it powers pumps or turbines for brine circulation anddisposal. When mechanical pumps or turbines are used, several pumps orturbines can be operated at different depths in the wellpipe to recovernatural gas from different portions of the methane-containing brine.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic drawing of a recovery device for the brinecirculation and recovery of natural gas.

FIG. 2 shows a schematic drawing of the starting mechanism for the brinecirculation and recovery system.

FIG. 3 shows the essential elements of a self-powered, piston-driven,pumping system for circulating brine and removing dissolved methanewhile suppressing steam vaporization and precipitation of solids.

FIG. 4 shows essential elements of a self-powered, turbine-driven,pumping system for circulating brine and removing dissolved methane.

FIG. 5 shows three pumping systems as shown in FIG. 3 in differentregions of a wellpipe operating simultaneously to remove methane fromsolution.

DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 1 a wellpipe 1 penetrates an ocean 2 and deep, sedimentarystrata 3. Within the wellpipe 1 there is an inner pipe 4 which forms ajoint 5 to the wellpipe 1 at some convenient depth. Inner pipe 4 is usedto establish four regions within the wellpipe. In the first regionvirgin brine containing dissolved methane flows into the bottom 6 of thewellpipe and moves up the wellpipe 1. Filters and well perforations ofcustomary design (not shown) are used to increase the brine flow andremove solids. As the virgin brine moves up the wellpipe, the pressurehead from the column of fluid is reduced, and some small methane bubblesform. As the methane-containing brine rises and moves into the innerpipe 4, the bubbles become larger and significantly affect the amount ofbrine which can be held in the inner pipe 4 to make up the pressurehead. This region of bubbly brine 7 ends where brine spills out of thetop 8 of the inner pipe 4. A region of relatively bubble free brine 9circulates out of the wellpipe 1 at perforations 10, and brine in theporous formations moves to equalize the pressure inhomogeneities createdwithin the formations by the brine circulation. The circulation rate ofthe brine reflects a number of factors including the height 11 of thebrine column between the inner pipe 4 and the wellpipe 1, the differencein pressure heads from the top 8 of the bubbly-brine 7 and the top 11 ofthe bubble free brine 9, the permeability of the formations, and thepressure exerted by the natural gas in the upper region 12 of thewellpipe. This gas pressure is maintained higher than the pressure ofsaturated steam over the brine; for brine at 300° F., this gas pressureis greater than about 65 psia which is the saturated pressure of steamover pure water in that temperature. The height of the bubble free brine11 will find a location which balances the production of virgin brine atthe bottom 6 of the wellpipe 1 and the reinjection of spent brine at theperforations 10. The height 11 can be above or below the top 8 of theinner pipe 4. The condition for steady-state production of brine andrecovery of natural gas involves a pressure of bubbly brine 7 plus gas12 which is less than the hydrostatic pressure or geopressure of theformation at the bottom 6 while the pressure of bubble free brine 9 plusgas 12 is sufficient to overcome the formation pressure at the dischargeperforations 10. These pressures exist because the release of gasbubbles from the virgin brine acts as a gas lift for the upward-movingbrine, and the expansion of the released gas lifts the brine to a higherheight 11 than it would rise against the ambient gas pressure 12 if thegas lift were not taking place. Natural gas product is delivered throughrelease valve 13 at about the operating pressure which was selected tosuppress steam vaporization and reduce solid precipitation. However,some solid precipitation will occur because of cooling of the hot brineas it moves up the lower portion of wellpipe 1 and through the innerpipe 4. Therefore, means must be provided to remove this solid afterimportant amounts of it have built up inside the wellpipe 1 and innerpipe 4. This means is provided by a cap 14 on the wellpipe 1 whichprovides for insertion of scrapers, reamers, scouring solutions, etc.,which will remove the built-up deposits. In addition, the cap 14provides means to introduce and attach a priming system to startcirculation.

In FIG. 2 a startup pipe 15 is introduced through cap 14 and attached toinner pipe 4 at its top 8. Gas is blown into the startup pipe 16displacing the brine with gas 17 throughout the inner pipe 4 and thelower portion of the wellpipe 1. Bubbles 18 flow into the surroundingformation 3 when the brine level has dropped to the bottom 6 of thewellpipe 1. To initiate circulation, the startup pipe 15 is lifted orremoved, and the gas pressure is released, permitting methane-containingbrine to rise in the lower wellpipe 1 and inner pipe 4, meanwhilereleasing methane, and causing circulation to achieve steady state byadjusting itself to ambient conditions. Circulation can be halted at anytime by slowly releasing the internal gas pressure to atmosphericthrough release valve 13. Under these conditions the gas lift isdestroyed, and the original quiescent conditions are achieved downhole.If the wellpipe is broken off by some accident, the uncontrolled releaseand ejection of brine will locally deplete the brine around the bottom 6of the wellpipe 1 and halt the flow. For hydrostatically pressuredbrines the flow of cold brine into the wellpipe will quench theself-powered circulation.

In FIG. 3 a self-powered, piston-driven, pumping system 19 is emplacedin a wellpipe 20, being attached by top and bottom seals 21.Perforations 22 in the lower section of the wellpipe 20 permit virginbrine 23, at hydrostatic pressure (or geopressured) and containingdissolved methane, to flow into the pumping system 19. A first pump 24is connected to a second pump 25 and, because the cylinders and pistonsin each pump are similar, each pump will pass equal volumes of liquid.Pump 24 permits entry of virgin brine into the pumping system 19 whilethe second pump 25 moves methane-free, spent brine 26 throughperforations 27 in the wellpipe 20 into the disposal region of the brinyformation. The work obtained by decompression of virgin brineessentially equals the work of recompression required to inject spentbrine into the disposal formation. In the pumping system of thisinvention, most or all of the work necessary to overcome friction isderived through the release of methane and limited amounts of steam andthe expansion of this gas against a piston in a third pump region 28. Byconventional techniques (not shown) additional work could be done by theuse of pressurized fluids delivered from the surface to the down holepumps or by electric motors powered from the surface. Also, work can bedone if geopressured brines are delivered to the pumps and the brine isinjected into hydrostatically pressured regions. The work of the gasexpansion in third pump 28 applies pumping pressure to spent brine in afourth pump 29 which supplies spent brine to the injection pump 25 fromwhich it moves to the disposal region of the formation. After themethane has been largely released from the brine by third pump 28, themethane and spent brine move into region 30 where the gas moves up tothe surface through pipe 31 and the spent brine moves to region 32 whichis a reservoir for pumps 29 and 25. The walls of the pumping system 19and the methane delivery pipe 31 must be capable of sustaining crushingpressures of several thousand psia because outside pressures (in thisexample) are about 7000 psia as at regions beside the wellpipe and areabout 100 psia at regions 30, 31, and 32. The circulation of virginbrine is started by pumping gas into the methane-delivery pipe 31 anddisplacing brine out of the system. Likewise circulation can be startedby auxiliary power from the surface. The circulation can be stopped bypouring brine into the methane delivery pipe 31.

In FIG. 4 a coupled, double turbine 41 with outer section 42 is drivenby virgin brine 23 which moves into the wellpipe 34 from entranceperforations 22. The inner section 36 of the turbine 41 pumpsmethane-free spent brine 37 into a disposal formation throughperforations 38. The pressure in the outer section 42 of the turbine 41drops from formation pressure of about 7000 psia in this example at thebottom 39 of the turbine 41 to a methane release pressure of about 100psia in this example at the top 40 of the outer section 42. Expansion ofmethane and small amounts of steam takes place in the outer section 42of the turbine 41, but the corresponding pumping for injection of thespent brine into the disposal formation involves only a trivialcompression of the brine, and even this work of compression is balancedin release of the virgin brine. Therefore, the spent brine can be pumpedfrom about 100 psia to about 7000 psia with minimal work, and the fluidrelease of the virgin brine is able to do enough work to overcome thefrictional losses in the turbines and in the flow in the formation.Released methane moves to recovery through the methane delivery pipe 31.The double turbine 41 is sealed into the wellpipe 34 by top and bottomseals 21. The pumping system must withstand a pressure drop of about7000 psia between the formation pressure and the pressure at which themethane is delivered to the surface. Additional power can be suppliedfrom the surface. The circulation is started either by pumping gas intothe methane-delivery pipe 31 and displacing brine out of the system orby operating the pumps from surface power. Circulation is stopped bypouring brine into the methane-delivery pipe 31.

In FIG. 5 is shown the multiple use of pumping systems 46, 47, 48 of thetype described in FIGS. 3 or 4 in which a wellpipe 1 penetrates asection of ocean 2 and methane-containing formations 3. Lower 46, middle47, and upper 48 pumping systems are each independently accepting virginbrine 23 through perforations 22 and independently delivering spentbrine through perforations 27 to disposal, while supplying methane to acommon delivery pipe moving gas to the surface at low pressure relativeto the adjacent formation pressure. The methane pressure is set by pipesizes and delivery rates, but it is low enought to release most of themethane from the virgin brine while suppressing the steam vaporizationand delivering an adequate supply of methane to the surface.

The foregoing description of a preferred embodiment of the invention hasbeen presented for purposes of illustration and description and is notintended to be exhaustive or to limit the invention to the precise formdisclosed. It was chosen and described in order to best explain theprinciples of the invention and their practical application to therebyenable others skilled in the art to best utilize the invention invarious embodiments and with various modifications as are suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto.

What is claimed is:
 1. A method of recovering natural gas from solutionin hot brines in which circulation of the brine and recovery of thedissolved natural gas is powered by the expansion of natural gasreleased from solution in the brine comprising: (a) injecting a gas intoa wellpipe to displace brine, (b) releasing the injected gas, (c)allowing methane-containing brine to flow into a pumping system therebyreleasing the natural gas, (d) maintaining a pressure of natural gasgreater than the vapor pressure of saturated steam over the brine atformation temperature, and (e) injecting the spent brine back into aformation.
 2. The method of claim 1 in which solids precipitated bycooling of methane-containing brines are removed through a cap at thetop of the wellpipe that allow ready access to the precipitated solids.3. The method of claim 1 in which circulation in the said pumping systemis stopped by slow release of methane out of the wellpipe.
 4. The methodof claim 1 in which the said pumping system comprises two or morecoupled pumps of turbines operating within a wellpipe simultaneously,and (a) accepting steam-saturated, methane-containing brine from aporous, subsurface formation, (b) using the said brine as a workingfluid in a first pump or turbine, (c) expanding the fluid by releasingdissolved natural gas, (d) causing the expanding fluid to supply all ora portion of the work required to operate a second pump or turbine whichreinjects the spent brine into a disposal formation, and (e) deliveringthe released gas to the surface for recovery.
 5. The method of claim 4in which two or more pumping systems of coupled pumps or turbines areoperated simultaneously, with each system being independently fedmethane-containing brine from different regions adjacent to a singlewellpipe.
 6. The method of claim 4 in which circulation in the saidpumping system is stopped by adding brine to the gas delivery pipe. 7.The method of claim 4 in which the coupled pumps of turbines injectingthe spent brine back into the formation are driven by the release of gasin the geopressured brine.